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A Mars-to-earth rock-throwing method.

A Mars-to-earth rock-throwing method

Sincethe late 1970s, a growing number of scientists have been studying the possibility that a tiny fraction of the 10,000 or so meteorites known on earth may have come from Mars. Colloquially known as "snicks," or SNCs, the intriguing space rocks are named for the acronym of three meteorites--Shergotty, Nakhla and Chassigny. Fragments of thsoe three are different in composition from all other studied meteorites as well as from the earth and moon, and some bear gas-isotope ratios consistent with atmospheric measurements made in 1976 by the Viking landing craft on Mars.

Such data notwithstanding, however, a key question has been how such chunks could have been kicked free of Mars's gravity to get hee in the first place. The possibility got a boost in 1983 when more than 20 teams of scientists concluded almost unanimously that a different meterorite had come from earth's moon (SN:3/26/83, p. 196), but the escape velocity required to leave Mars is about twice as great. Even with geochemical evidence seeming to link Marsh andthe SNCs, is such a feat "mechanically" possible?

Now a pair of Caltech researchers has concluded that the answer is yes.

Merely hitting the Martian surface with an incoming meteorite sufficiently fast and massive to knock loose some pieces is not good enough. Under many conditions, various researchers have concluded, such an impact would partially melt or even vaporize any escaping fragments, yet two of the three kinds of SNCs seem to have been only lightly to moderately shocked by whatever set them free.

One possible implication of such an inconsistency has always been that perhaps the SNCs did not come from Mars at all, leaving the geochemical findings as an increasingly frustrating coincidence. But according to John D. O'keefe and Thomas J. Ahrens of the Caltech Seismological Laboratory in Pasadena, there is a less shocking means of escape.

The answer, the authors propose in the Oct. 17 SCIENCE, is an oblique impact by the incoming object, vaporizing enough of itself to form high-speed vapor jets or plumes that can carry away, or entrain, surface rocks and accelerate them to speeds faster than Martian escape velocity. The Caltech team combined computer simulations with actual tests in a compressed-gas gun that fired solid plastic projectiles. At impact angles of 25 degrees to 60 degrees from the horizontal, the tests showed vapor plumes moving at speeds of almost 50,000 miles per hour. On Marsh, the researchers calculated, incoming projectiles (meteorites) from 0.12 to 1.2 miles in diameter could produce plumes dense enough (0.1 to 1.0 grams per cubic centimeter) to pick up surface rocks and drive them away at speeds sufficient to send them all the way to earth.

In fact, report O'Keefe and Ahrens, "oblique impact-induced jet plume entrainment appears to be the only mechanism that provides the physical mechanism required to explain the acceleration to high speed of lightly shocked planetary samples such as SNC meteorites."

the confirmation of the lunar meteorites (the tentatively identified number is now up to three) was made possible, however, only by the presence of samples from the Apollo program for comparison. Now, about a Mars sample-return mission . . .
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Title Annotation:possibility that some meteorites may come from Mars
Author:Eberhart, Jonathan
Publication:Science News
Date:Oct 18, 1986
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